Conventionally, there is proposed an active vibration noise control device for controlling an engine sound heard in a vehicle interior by a controlled sound output from a speaker so as to decrease the engine sound at a position of passenger's ear. Concretely, noticing that a vibration noise in a vehicle interior is generated in synchronization with a revolution of an output axis of an engine, there is proposed a technique for canceling the noise in the vehicle interior on the basis of the revolution of the output axis of the engine by using an adaptive notch filter so that the vehicle interior becomes silent.
This kind of technique is proposed in Patent Reference 1, for example. In Patent Reference 1, there is proposed an active vibration noise control device for fading a control sound out when a filter coefficient is larger than an upper limit (first threshold) predetermined number of times, and for starting an adaptive controlling process again when the filter coefficient is smaller than a lower limit (second threshold). The technique aims to prevent an occurrence of an abnormal sound which occurs when a sound detector (for example microphone) is covered.
Additionally, there is disclosed a technique related to the present invention in Patent Reference-2.